Windshields for motor vehicles have been known which have a decreased transmission of the visible spectrum in the upper area of the windshield to serve as a protection against the direct dazzling of the driver by incident sun rays from above and at the same time as a protection against heat. In the case of laminated glass panes, this area of reduced transmission customarily is achieved by the use of thermoplastic intermediate layers having homogenously or continuously dyed strips to form a band-pass filter.
Beside the direct dazzling of the driver during the day caused by incident light of the sun low on the horizon or at night by headlights of oncoming vehicles, the vision of the driver may also be impaired by indirect dazzling. Hitherto, little consideration has been given to the significance of indirect dazzling as brought about, for example, by the reflecting and light scattering surfaces of the roadway directly in front of the vehicle, by the hood of the vehicle or by a light scattering windshield. No effective arrangements are known which are suitable to intentionally decrease the disturbing factors caused by indirect dazzling.
Light striking a transparent pane, for example, a windshild, does not always pass through at the angle of refraction or at the angle of reflection, since some deviation of the direction of light can be expected to occur as a result of light scatter and through diffraction of light. The windshield of a vehicle may be considered as an illuminated light scattering body which, especially at night, leads to impediments of sight as a result of haze glare. Haze glare not only occurs in the case of strongly acting windshields, as is well known, by direct incident light, but as experiments have shown, also by the light of a vehicle's own dimmed headlights reflecting from the roadway and by the light of the street illumination reflected on the hood of the vehicle.
For the visibility of an object, the relative difference of luminance between the object and the outer field (photometric contrast) is decisive. In this case, the following formula is valid: ##EQU1## The haze luminance of a scattering transparent pane is superposed both on the value B.sub.u as well as on the value B.sub.o in the case of haze and fog dazzle. In that case, the following is valid: ##EQU2## K.sub.s in this case always assumes smaller values than K. The capacity of the eye to discern differences in luminance in the case of a certain illumination level requires a minimum value of K. K is proportional to the distance of vision. K&gt;K.sub.s always signifies a decrease of the distance of vision.
In the practical operation of a vehicle, windshields always have some surface roughening, scratches and nicks which act as light scattering centers. In the same manner scattered light is caused by dust particles, dirt and coatings of moisture on the surface of the pane. The glass material itself, and in the case of windshields made from laminated glass panes, the layers of plastic between the panes are each considered as not being optically empty. Portions of scattered light of about 0.4% have been measured in panes and in laminated panes because of contamination of the glass or plastic layers or due to inclusion of small foreign bodies in the glass panes or plastic layers.
A number of tests have been made which led to the result that a high correlation exists between the length of use of a windshield and its scatter light factor. With increasing length of use, the amount of the surface damage of the panes caused by the abrasion process due to the action of the windshield wipers increases. Also the surface of the panes become pitted as a consequence of being struck by stones. In the case of windshields with operational distances of 100,000 km, scatter light values of 2 to 4% have been measured. Dusty or dirty windshields or windshields coated on the inside with nicotine or softener condensates may cause additional values at the same order of magnitude. In the case of moisture condensations on the inside or in the case of a water film on the outside of a windshield, values of scattered light up to 15% have been measured.
A light scattering windshield acts in effect as a source of light or as an illuminated pane which reflects diffuse light. The dazzle effect developed depends on the intensity of the stream of light striking the pane, on the manner and the dimension of the scattering media, and likewise on the size of the surface of the pane.
For the haze glare viewpoint of the driver, the light of his own vehicle's headlights, and the street illumination reflected by the roadway and the hood of his car are all of significance and a cause of indirect light striking a light scattering windshield.
The level of illumination occurring in the area close to the vehicle, and especially when halogen lights are used to provide the illumination, causes an intensive reflection. This is particularly true in the case of a strongly reflecting surface of a roadway where the reflection may be considered as a cause for the formation of haze luminance, not only in connection with light scattering windshields, but generally may represent a direct cause for relative dazzle or adaption dazzle and even absolute dazzle depending on the strength of the reflection.
It is therefore an object of the invention to provide for an anti-glare pane for a vehicle which will reduce haze glare and also reduce indirect glare or relative glare caused by light penetrating the parts of the pane outside of the main field of vision and especially light reflected from the hood and the roadway in the area close to the vehicle.